Assuming your branch with the bad commit(s) has already been published to GitHub, and other users may have already pushed it, then you typically would not literally rollback that branch. Instead, you would use git revert
to functionally undo the bad commit:
git revert 1a7h3jxk # where 1a7h3jxk is the SHA-1 hash of the bad commit
git revert
actually adds a new commit which functionally undoes the commit you specify as a parameter. It also allows syntax for specifying a range of commits.
If you really want to formally "backtrack" your branch, you could do a hard reset to remove the bad commit(s). For example, to actually remove a single bad commit you could do:
git reset --hard HEAD~1
git push --force origin your_branch
But note carefully that doing a hard reset means that you are rewriting the history of your branch. This means you have to force push (read: overwrite) the version of the branch on the remote. As mentioned above, this option should not be used if anyone shared this branch besides you.